there is a common problem in the computers of older mitsu's and especially
dsm's, where an electronic component fails and leaks caustic crap on to the
printed circuit board. if i am not mistaken, the traces on the circuit board
for the fuel injectors often get "eaten" when this problem occurs.
your computer is in a metal box in your center console. pull it out and open
the box. you will most likely see traces of corrosion surrounding the three
cylinder shaped electrolytic capacitors.
$100 is common for a used ecu. anything that has not been "re-capped" will
need to be serviced. the parts are $1.50 and if you catch it before the
board is damaged it is pretty easy to fix.
http://www.tmo.com/howto/ecu1g/caps.htm
http://www.foreignecurepair.com/